/*
 * Copyright 2002-2014 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package qunar.tc.bistoury.instrument.client.spring.el;


/**
 * Represents a reference to a value.  With a reference it is possible to get or set the
 * value. Passing around value references rather than the values themselves can avoid
 * incorrect duplication of operand evaluation. For example in 'list[index++]++' without a
 * value reference for 'list[index++]' it would be necessary to evaluate list[index++]
 * twice (once to get the value, once to determine where the value goes) and that would
 * double increment index.
 *
 * @author Andy Clement
 * @since 3.2
 */
interface ValueRef {

    /**
     * Returns the value this ValueRef points to, it should not require expression
     * component re-evaluation.
     *
     * @return the value
     */
    TypedValue getValue();

    /**
     * Sets the value this ValueRef points to, it should not require expression component
     * re-evaluation.
     *
     * @param newValue the new value
     */
    void setValue(Object newValue);

    /**
     * Indicates whether calling setValue(Object) is supported.
     *
     * @return true if setValue() is supported for this value reference.
     */
    boolean isWritable();


    /**
     * A ValueRef for the null value.
     */
    static class NullValueRef implements ValueRef {

        static final NullValueRef INSTANCE = new NullValueRef();

        @Override
        public TypedValue getValue() {
            return TypedValue.NULL;
        }

        @Override
        public void setValue(Object newValue) {
            // The exception position '0' isn't right but the overhead of creating
            // instances of this per node (where the node is solely for error reporting)
            // would be unfortunate.
            throw new SpelEvaluationException(0, SpelMessage.NOT_ASSIGNABLE, "null");
        }

        @Override
        public boolean isWritable() {
            return false;
        }
    }


    /**
     * A ValueRef holder for a single value, which cannot be set.
     */
    static class TypedValueHolderValueRef implements ValueRef {

        private final TypedValue typedValue;

        private final SpelNodeImpl node;  // used only for error reporting

        public TypedValueHolderValueRef(TypedValue typedValue, SpelNodeImpl node) {
            this.typedValue = typedValue;
            this.node = node;
        }

        @Override
        public TypedValue getValue() {
            return this.typedValue;
        }

        @Override
        public void setValue(Object newValue) {
            throw new SpelEvaluationException(this.node.pos, SpelMessage.NOT_ASSIGNABLE, this.node.toStringAST());
        }

        @Override
        public boolean isWritable() {
            return false;
        }
    }

}
